Approximating topology of optically writable medium surface

ABSTRACT

A method controls an assembly using an oscillating wave, such that the assembly moves in an oscillated manner in relation to a surface of an optically writable medium in correspondence with oscillation of the oscillating wave. The assembly generates a beam towards the surface of the optically writable medium. A proximity signal is detected in response to the beam being generated towards the surface of the optical medium, and denotes closeness of the assembly to the surface. A number of peaks within the proximity signal are determined. The time at which each peak within the proximity signal occurs is correlated with a value of the oscillating wave at that time, yielding a number of time-value pairs. The topology of the surface of the optical medium is approximated from these time-value pairs.

RELATED APPLICATIONS

The present patent application is a continuing patent application of andclaims priority to the pending U.S. patent application entitled“Approximating topology of optical disc surface,” filed on Sep. 15,2003, and assigned Ser. No. 10/662,712 now U.S. Pat. No. 7,239,587.

BACKGROUND OF THE INVENTION

Computer users employ writable and rewritable optical discs for avariety of different purposes. They may save programs or data to thediscs, for archival or distribution purposes. In the case of CD-typediscs, users may make music CD's that can be played in audio CD players,or save music data files to the CD's, such as MP3 files, that can beplayed in special-purpose CD players. In the case of DVD-type discs,users have greater storage capacity available to them than with CD-typediscs, and may be able to make video DVD's that can be played instand-alone DVD players.

Many types of optical discs include a data side and a label side. Thedata side is where the data is written to, whereas the label side allowsthe user to label the optical disc. Unfortunately, labeling can be anunprofessional, laborious, and/or expensive process. Markers can be usedto write on optical discs, but the results are decidedly unprofessionallooking. Special pre-cut labels that can be printed on with inkjet orother types of printers can also be used, but this is a laboriousprocess: the labels must be carefully aligned on the discs, and so on.Special-purpose printers that print directly on the discs may be used,but such printers are fairly expensive. In the patent applicationentitled “Integrated CD/DVD Recording and Label”, filed on Oct. 11,2001, and assigned Ser. No. 09/976,877, a solution to these difficultiesis described, in which a laser is used to label optical discs.

When reading or writing from the label or data side of an optical disc,the surface of the optical disc may be presumed to be perfectly flat.However, in actuality, the optical disc surface is typically slightlywarped, having a surface topology that can be generally likened to thatof a potato chip, albeit with considerably less warping as compared to apotato chip. Although this slightly warped nature of the optical discsurface may be difficult if not impossible to discern with the nakedeye, it can affect reading and writing label markings and data on thesurface. Performance in reading and writing may be reduced, and qualitymay be impaired.

SUMMARY OF THE INVENTION

A method of one embodiment of the invention outputs an oscillating wavetowards a surface of an optical disc. A proximity signal is detected inresponse to the oscillating wave being output towards the surface of theoptical disc, and denotes closeness of the oscillating wave to thesurface. A number of peaks within the proximity signal are determined.Each peak corresponds to the oscillating wave crossing the surface ofthe optical disc. The time at which each peak within the proximitysignal occurs is correlated with a value of the oscillating wave at thattime, yielding a number of time-value pairs. The topology of the surfaceof the optical disc is approximated from these time-value pairs.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings referenced herein form a part of the specification.Features shown in the drawing are meant as illustrative of only someembodiments of the invention, and not of all embodiments of theinvention, unless otherwise explicitly indicated.

FIG. 1 is a diagram of an optical disc having an optically writablelabel side with a number of tracks, according to an embodiment of theinvention.

FIG. 2 is a diagram of an optical disc surface-tracking assembly,according to an embodiment of the invention.

FIG. 3 is a graph depicting how a drive signal intersecting with thesurface of an optical disc yields peaks in a proximity signal, accordingto an embodiment of the invention.

FIG. 4 is a graph depicting how an oscillating wave output towards thesurface of an optical disc can approximate the topology of the opticaldisc surface, according to an embodiment of the invention.

FIG. 5 is a flowchart of a method for using an oscillating wave and theresulting proximity signal to approximate the topology of the surface ofan optical disc, according to an embodiment of the invention.

FIG. 6 is a diagram of a mass storage device, according to an embodimentof the invention.

FIG. 7 is a flowchart of a method for manufacturing the mass storagedevice of FIG. 6, according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following detailed description of exemplary embodiments of theinvention, reference is made to the accompanying drawings that form apart hereof, and in which is shown by way of illustration specificexemplary embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilized,and logical, mechanical, and other changes may be made without departingfrom the spirit or scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims.

Optical Disc and Optical Disc Surface-Tracking Assembly

FIG. 1 shows an optical disc 100, according to an embodiment of theinvention. The optical disc 100 may be a compact disc (CD), a digitalversatile disc (DVD), or another type of optical disc. The optical disc100 has an optically writable label side 102, which is shown in FIG. 1,and an optically writable data side 104 on the opposite side of theoptical disc 100. An example of the optically writable label side 102 isparticularly disclosed in the patent application entitled “IntegratedCD/DVD Recording and Label”, filed on Oct. 11, 2001, and assigned Ser.No. 09/976,877. An image may be written to the label side 102, such thatthe pixels of the image are selectively and correspondingly opticallywritten to the label side 102.

The label side 102 is more generally a label region, and the data side104 is more generally a data region, in that an optically writable labelregion may coexist on the same side of the optical disc 100 as anoptically writable data region. The optical disc 100 also has an insideedge 106 and an outside edge 108. The optical disc 100 also has a numberof concentric circular tracks 110A, 110B, . . . 110N, collectivelyreferred to as the tracks 110. There may be hundreds, thousands, or moreof the tracks 110. Alternatively, the tracks 110 may be spiral tracks.An optical marking mechanism, such as a laser, may write marks, orpixels, to marking, or pixel, positions of the tracks 110. The opticalmarking mechanism accomplishes this by moving over each of the tracks110, such that the optical disc 100 is rotated so that the opticalmarking mechanism is able to pass over the entirety of each of thetracks 110.

FIG. 2 shows an optical disc surface-tracking assembly 200, according toan embodiment of the invention. The assembly 200 may be used to trackeither the optically writable label surface 102 or the opticallywritable data surface 104 of the optical disc 100, as well as othertypes of surfaces of optical discs. Furthermore, the assembly 200 maymore generally be a reflective surface-tracking assembly, used fortracking a reflective surface other than an optical disc surface. Theassembly 200 is depicted in FIG. 2 as including a controller 202, abeam-generating mechanism 204, and a sensing mechanism 206.

The optical disc surface-tracking assembly 200 is shown in FIG. 2 asbeing incident to the optically writable label surface 102 of theoptical disc 100. Both the label surface 102 and the optically writabledata surface 104 of the optical disc 100 are depicted in FIG. 2 as beingwarped. The degree to which the surfaces 102 and 104 are warped isexaggerated in FIG. 2 for illustrative clarity.

In general, the optical disc surface-tracking assembly 200 operates asfollows. The beam-generating mechanism 204 outputs a beam 210, such asan optical beam, which is reflected off the surface 102 of the opticaldisc 100. The sensing mechanism 206 detects the reflected beam 214 offthe surface 102. The beam-generating mechanism 204 provides a drivesignal 212 to the controller 202, corresponding to the beam 210, and thesensing mechanism 206 provides a proximity signal 216 to the controller202, corresponding to the reflected beam 214. In response, thecontroller 202 determines the surface topology 218 of the surface 102that is incident to the beam 210.

More specifically, the beam-generating mechanism 204 outputs the beam210. The beam-generating mechanism 204 is moved closer to and fartheraway from the label surface 102 of the optical disc 100, as indicated bythe bi-directional arrow 208. The movement of the mechanism 204 is inaccordance with a desired waveform, such as a sinusoidal wave, atriangle wave, a saw tooth wave, or another type of periodic waveform.The drive signal 212 is thus an oscillating wave. The beam-generatingmechanism 204 includes those components that allow for the generation ofan optical or other type of beam.

In one embodiment, the beam-generating mechanism 204 is itself movedcloser to and farther away from the label surface 102 of the opticaldisc 100. In this embodiment, the focusing lens of the mechanism 204through which the optical beam is output is fixed, such that the entiremechanism 204 is moved closer to and farther away from the surface 102to change the focus in accordance with an oscillating wave, as the drivesignal 212. In another embodiment, the focusing lens of the mechanism204 is the only component of the mechanism 204 that moves closer to andfarther away from the surface 102, as indicated by the arrow 208, tochange the focus in accordance with the oscillating wave, as the drivesignal 212.

The sensing mechanism 206 detects the beam 214, which is the beam 210that is reflected off the label surface 102 of the optical disc 100. Inresponse, the sensing mechanism 206 generates the proximity signal 216.The proximity signal 216 is the value of the beam as reflected off thesurface 102 and detected by the sensing mechanism 206. In particular,the proximity signal 216 is a measure of the absolute closeness of theoscillating wave that is the beam 210 to the surface 102. The beam 210is oscillated such that it can overshoot the surface 102, as well as maynot extend far enough to impinge the surface 102. The proximity signal216, therefore, indicates how close the beam 210, and thus thecorresponding drive signal 212, is to the surface 102 of the opticaldisc 100. The sensing mechanism 206 includes those components that allowfor the sensing of the reflected beam 214, and may be an optical oranother type of sensor.

For occurrences of the beam 210 crossing the label surface 102 of theoptical disc 100, the proximity signal 216 denotes such crossing withlocal maximums, or peaks. That is, peaks or local maximums within theproximity signal 216 indicate the times, and thus positions since thebeam 210 moves across the surface 102 over time, at which the beam 210crosses the label surface 102. Because the value of the drive signal 212is known at these times or positions, the controller 202 is able toapproximate the topology of the label surface 102 from the times atwhich peaks or local maximums within the proximity signal 216 occur, andfrom the values of the drive signal 212 at these times. This isdescribed in more detail in the next section of the detaileddescription. By approximating the topology of the surface 102, thecontroller 202 is thus able to track the surface 102 of the optical disc100.

Approximating Topology of Optical Disc Surface

FIG. 3 shows a graph 300 depicting the relationship between adisturbance 306, a drive signal 308, and a sense signal 310, accordingto an embodiment of the invention. The disturbance 306 can represent theslightly warped or otherwise irregular surface of an optical disc, suchas the label surface 102 or the data surface 104 of the optical disc100. The drive signal 308 is the drive signal 212 in FIG. 2, and is thecase where oscillation of the drive signal 308 does not occur, for sakeof simplicity. The sense signal 310 is the proximity signal 216 in FIG.2. The x-axis 304 of the graph 300 denotes time, to which position cancorrespond.

The y-axis 302 of the graph 300 denotes the values of the disturbance306, the drive signal 308, and the sense signal 310. For instance, inthe case of the disturbance 306, the value denoted on the y-axis 302 canbe the height of the surface of an optical disc from a flat referencerunning through the middle of the optical disc. In the case of the drivesignal 308, the value denoted on the y-axis 302 may be the intensity ofthe drive signal 308.

Furthermore, in the case of the sense signal 310, the value denoted onthe y-axis 302 may be a measure of the closeness of the drive signal 308to the disturbance 306. Higher values within the sense signal 310indicate that the drive signal 308 is closer to the disturbance 306, andpeaks or local maximums within the sense signal 310 indicate that thedrive signal 308 has crossed the disturbance 306. Thus, in the graph 300of FIG. 3, in the two places where the drive signal 308 has crossed thedisturbance 306, local maximums or peaks result in the sense signal 310.

FIG. 4 shows a graph 400 indicating how the drive signal 308 and thesense signal 310 can be used to approximate the disturbance 306 as theapproximation 312, according to an embodiment of the invention. Asbefore, the disturbance 306 can represent the slightly warped orotherwise irregular surface of an optical disc, such as the labelsurface 102 or the data surface 104 of the optical disc 100. The drivesignal 308 is the drive signal 212 in FIG. 2, and is the case whereoscillation of the drive signal 308 occurs, resulting in an oscillatingwave, as has been described. The sense signal 310 is the proximitysignal 216 in FIG. 2. The x-axis 304 of the graph 300 denotes time, towhich position can correspond, and the y-axis 302 of the graph 300denotes the values of the disturbance 306, the drive signal 308, thesense signal 310, and the approximation 312.

As before, for each occurrence in which the drive signal 308 crosses thedisturbance 306, there is a corresponding peak or local maximum withinthe sense signal 310. The times, or positions, on the x-axis 304 atwhich these peaks within the sense signal 310 occur are determined, andare correlated with the corresponding values of the drive signal 308 atthese times or positions. This yields a number of time-value, orposition-value, pairs, from which the approximation 312 is thendetermined. These pairs are indicated in FIG. 4 as the points 452A,452B, . . . , 452N. For example, a curve-fitting approach can beemployed to yield the approximation 312 from the time-value pairs. Onesuch curve-fitting approach is a beta-spline. In another curve-fittingapproach, the time-value pairs are translated into uniform time samples,such as by linear or another type of interpolation, and then aretranslated into the frequency domain by a Fourier transform. The loworder coefficients in the frequency domain corresponding to the initialharmonics, such as the first three, are then used to generate theapproximation 312.

FIG. 5 shows a method 500 for approximating the topology of an opticaldisc surface, or another type of reflective surface, according to anembodiment of the invention. The method 500 may be implemented as acomputer program stored on a computer-readable medium. The medium may bea volatile or a non-volatile medium. The medium may also be a magneticmedium, such as a tape cartridge, a floppy disk, or a hard disk drive,an optical medium, such as an optical disc, and/or a semiconductormedium, such as random-access memory or flash memory. The medium may bepart of or accessed by the controller 202 of the assembly 200 of FIG. 2,such that the controller 202 and the assembly 200 can be considered tobe performing the method 500.

First, an oscillating wave is generated (502), and output towards asurface of an optical disc (504). The oscillating wave may be theoptical beam 210 of FIG. 2 that results from the beam-generatingmechanism 204 outputting the optical beam 210 while oscillating to andfrom the label surface 102 of the optical disc 100, as indicated by thebidirectional arrow 208 in FIG. 2. The surface of the optical disc isslightly warped, or is otherwise an irregular surface, and may be theoptically writable label side 102 of the optical disc 100, the opticallywritable data side 104 of the optical disc 100, or another type ofoptical disc surface.

A proximity signal is detected in response to the oscillating wave beingoutput towards the optical disc surface (506). For instance, the beam214 reflected off the optical disc surface 102 of the optical disc 100in FIG. 2 may be detected, such that the proximity signal is a value ofthis beam 214. As has been described, the peaks or local maximums withinthe proximity signal are determined (508), where each peak correspondsto the oscillating wave crossing the optical disc surface. The time atwhich each peak within the proximity signal occurs is correlated with avalue of the oscillating wave at that time (510), to yield a number oftime-value pairs. The time-value pairs may also be consideredposition-value pairs, since the oscillating wave is moving over thesurface of the optical disc surface as a function of time.

The topology of the optical disc surface is finally approximated fromthe time-value pairs (512). In one embodiment, a curve may becurve-fitted onto the time-value pairs to approximate the topology ofthe optical disc surface. A beta-spline curve-fitting approach can beused, and in one embodiment the Fourier transform can be used, toapproximate the topology of the optical disc surface from the time-valuepairs. Tracking of the optical disc surface can thus be accomplishedutilizing the topology of the surface as has been approximated.

Mass Storage Device

FIG. 6 shows the mass storage device 600, according to an embodiment ofthe invention. The mass storage device 600 is for reading from and/orwriting to the optical disc 100. More specifically, the mass storagedevice 600 is for reading from and/or writing to an optically writabledata region of the optical disc 100, and/or an optically writable labelregion of the optical disc 100. The mass storage device 600 includes abeam source 602A and an objective lens 602B, which are collectivelyreferred to as the optical marking mechanism 602. The storage device 600also includes a spindle 606A, a spindle motor 606B, and a rotary encoder606C, which are collectively referred to as the first motor mechanism606. The device 600 includes a sled 608A, a sled motor 608B, a linearencoder 608C, which is optional, and a rail 608D, which are collectivelyreferred to as the second motor mechanism 608. The motor mechanisms 606and 608 can be generally considered a movement mechanism. Finally, themass storage device 600 includes an optical disc surface-trackingassembly 610, which may be the surface-tracking assembly 200 of FIG. 2that has been described.

The optical marking mechanism 602 focuses an optical beam 604 on theoptical disc 100, for at least marking the label side 102 of the opticaldisc 100, and which also may be used to read from the label side 102 ofthe disc 100, as well as read from and/or write to the data side 104 ofthe disc 100. Specifically, the beam source 602A generates the opticalbeam 604, which is focused through the objective lens 602B onto theoptical disc 100, such as in a manner known to those of ordinary skillwithin the art.

The first motor mechanism 606 rotates the optical disc 100.Specifically, the optical disc 100 is situated on the spindle 606A,which is rotated, or moved, by the spindle motor 606B to a givenposition specified by the rotary encoder 606C communicatively coupled tothe spindle motor 606B. The rotary encoder 606C may include hardware,software, or a combination of hardware and software.

The second motor mechanism 608 moves the optical marking mechanism 602radially relative to the optical disc 100. Specifically, the opticalmarking mechanism 602 is situated on the sled 608A, which is moved onthe rail 608D by the sled motor 608B to a given position specified bythe linear encoder 608C communicatively coupled to the sled motor 608B.The linear encoder 608C may include hardware, software, or a combinationof hardware and software.

The surface-tracking assembly 610 controls the marking mechanism 602 andthe motor mechanisms 606 and 608 to cause markings, or pixels, to bewritten to pixel, or marking, positions on the tracks of opticalwritable label side 102 of the optical disc 100, such as in accordancewith an image to be written to the tracks of the label side 102. Thesurface-tracking assembly 610 may also control the marking mechanism 602and the motor mechanisms 606 and 608 to write data to the data side 104of the optical disc 100. The surface-tracking assembly 610 also causesthe motor mechanisms 606 and 608 to move the optical marking mechanism602 so as to track the optically writable label surface 102 or theoptically writable data surface 104 of the optical disc 100, as has beendescribed in the preceding sections of the detailed description.

In one embodiment, the surface-tracking assembly 610 is the surfacetracking assembly 200 of FIG. 2 that has been described. In thisembodiment, the beam-generating mechanism 204 is replaced by the opticalmarking mechanism 602, such that the marking mechanism 602 performs thefunctionality that has been described as being performed by thebeam-generating mechanism 204. The surface-tracking assembly 610,however, still includes the sensing mechanism 206 and the controller 202of FIG. 2, as has been described. The movement mechanism, inclusive ofthe motor mechanisms 606 and 608, oscillates the optical markingmechanism 602, as denoted by the bidirectional arrow 208 in FIG. 2.

Whereas FIG. 6 shows both a linear encoder 608C and a rotary encoder606C, other embodiments of the invention may not have either or both ofthe encoders 608C and 606C. For instance, the sled motor 608B may be astepper mode in which there is no closed-loop feedback, such that thelinear encoder 608C is not present. As another example, rotary encoder606C may not be present. In such case, either no rotary encoding isaccomplished, or the optical disc 100 itself serves as the encoder, viatiming marks printing thereon outside of any image areas.

As can be appreciated by those of ordinary skill within the art, thecomponents depicted in the mass storage device 600 are representative ofone embodiment of the invention, and do not limit all embodiments of theinvention. Other approaches can also be employed. As only one example,the sled 608A may be positioned with the sled motor 608B, with fineradjustment obtained using a voice coil attached to the beam source 602Aand/or the objective lens 602B.

FIG. 7 shows a method of manufacture 700 for the mass storage device 600of FIG. 6, according to an embodiment of the invention. The method 700includes providing the optical marking mechanism 602 (702), providingthe first motor mechanism 606 (704), providing the second motormechanism 608 (706), and providing the surface-tracking assembly 610(708). In one embodiment, providing the optical marking mechanism 602includes providing the optical beam source 602A (710) and the objectivelens 602B (712), whereas providing the first motor mechanism 606 in oneembodiment includes providing the spindle 606A (714), the spindle motor606B (716), and the rotary encoder 606C (718). Finally, providing thesecond motor mechanism 608 in one embodiment includes providing the sled608A (720), the sled motor 608B (722), and the linear encoder 608C(724).

Conclusion

It is noted that, although specific embodiments have been illustratedand described herein, it will be appreciated by those of ordinary skillin the art that any arrangement calculated to achieve the same purposemay be substituted for the specific embodiments shown. This applicationis intended to cover any adaptations or variations of the disclosedembodiments of the present invention. Therefore, it is manifestlyintended that this invention be limited only by the claims andequivalents thereof.

1. A method comprising: controlling an assembly using an oscillatingwave, such that the assembly moves in an oscillated manner in relationto a surface of an optically writable medium in correspondence withoscillation of the oscillating wave, the assembly generating a beamtowards the surface of the optically writable medium; detecting aproximity signal in response to the beam being generated towards thesurface of the optically writable medium, the proximity signal denotingcloseness of the assembly to the surface of the optically writablemedium; determining a plurality of peaks within the proximity signal;correlating a time at which each peak within the proximity signal occurswith a value of the oscillating wave at the time to yield a plurality oftime-value pairs; and, approximating a topology of the surface of theoptically writable medium from the plurality of time-value pairs.
 2. Themethod of claim 1, further comprising initially generating theoscillating wave.
 3. The method of claim 1, wherein detecting theproximity signal comprises detecting the beam reflected by the surfaceof the optically writable medium with the assembly, the proximity signaldetermined as a value of the beam reflected by the surface of theoptically writable medium over time.
 4. The method of claim 1, whereinthe surface of the optically writable medium is an irregular surface. 5.The method of claim 1, wherein the surface of the optically writablemedium is one of an optically writable label side and an opticallywritable data side of the optical disc.
 6. The method of claim 1,wherein the oscillating wave is one of a triangle wave, a saw toothwave, and a sinusoidal wave.
 7. The method of claim 1, whereindetermining the plurality of peaks within the proximity signal comprisesdetermining a plurality of local maximums within the proximity signal.8. The method of claim 1, wherein approximating the topology of thesurface of the optically writable medium from the plurality oftime-value pairs comprises curve-fitting a curve onto the plurality oftime-value pairs to approximate the topology of the surface of theoptically writable medium.
 9. The method of claim 8, whereincurve-fitting the curve onto the plurality of time-value pairs toapproximate the topology of the surface of the optically writable mediumcomprises using a beta-spline curve-fitting approach to approximate thetopology of the surface of the optically writable medium.
 10. The methodof claim 8, wherein curve-fitting the curve onto the plurality oftime-value pairs to approximate the topology of the surface of theoptically writable medium comprises using a number of coefficients of aFourier series of the plurality of time-value pairs to approximate thetopology of the surface of the optically writable medium.
 11. Acomputer-readable medium having a program stored thereon to perform amethod comprising: controlling an assembly using an oscillating wave,such that the assembly moves in an oscillated manner in relation to areflective surface in correspondence with oscillation of the oscillatingwave, the assembly generating a beam towards the reflective surface;receiving a proximity signal detected in response to the assemblygenerating the beam towards the reflective surface as the assembly movesin the oscillated manner, the proximity signal denoting closeness of theoscillating wave to the reflective surface; determining a plurality oflocal maximums within the proximity signal; correlating a time at whicheach local maximum within the proximity signal occurs with a value ofthe oscillating wave at the time to yield a plurality of time-valuepairs; and, approximating a topology of the reflective surface from theplurality of time-value pairs.
 12. The medium of claim 11, wherein theoscillating wave is one of a triangle wave, a saw tooth wave, and asinusoidal wave.
 13. The medium of claim 11, wherein approximating thetopology of the reflective surface from the plurality of time-valuepairs comprises curve fitting a curve onto the plurality of time-valuepairs to approximate the topology of the reflective surface.
 14. Themedium of claim 11, wherein the surface of the optically writable mediumis an irregular surface.
 15. The medium of claim 11, wherein the surfaceof the optically writ able medium is one of an optically writable labelregion and an optically writable data region of an optical disc.
 16. Asurface-tracking assembly comprising: a beam-generating mechanism togenerate a beam output towards a surface of an optically writablemedium, at least the beam-generating mechanism of the assembly beingoscillated to cause the beam to result in an oscillating wave; a sensingmechanism to detect the beam reflected by the surface of the opticallywritable medium to yield a proximity signal as a value of the beam asreflected and detected; and, a controller to track the surface of theoptically writable medium by approximating a topology of the surfacefrom a plurality of time-value pairs, each time-value pair including atime at which a peak within the proximity signal occurred and a value ofthe oscillating wave at the time.
 17. The assembly of claim 16, whereinthe surface of the optically writable medium is one of an opticallywritable label region and an optically writable data region of anoptical disc.
 18. The assembly of claim 16, wherein the beam comprisesan optical beam.
 19. The assembly of claim 16, wherein at least thebeam-generating mechanism of the assembly is oscillated in accordancewith one of a triangle wave, a square wave, and a sinusoidal wave. 20.The assembly of claim 16, wherein the controller approximates thetopology of the surface from the plurality of time-value pairs bycurve-fitting a curve onto the plurality of time-value pairs.